Isaac Newton accomplished some very important things in his life. He came up with a modern version of calculus (although not the modern notation). He described and explained a little thing called gravity. But he also is said to have invented a very useful thing for cats, in a very stupid way.

Most contemporary and biographical sources on Isaac Newton agree on to facts - he was a jerk, and he was brilliant. There's no disputing the second - or is there?

One modern legend has Isaac Newton inventing something that pet owners use all the time: the cat door. The legend is used to augment the sense of Newton's brilliance. He didn't just excel in the lofty realms of math and science, but in practical inventions as well. The aim of the modern version of the legend is encapsulated in a line from Douglas Adams' novel, Dirk Gently's Holistic Detective Agency. When one character says that the idea was simple and obvious, another replies, "It is an extraordinary mind which can take the previously non-existent and make it blindingly obvious."

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There's an irony to this, because not only is the cat door invention legend wrong, it sprang out of another legend - one that was circulated to show how stupid Newton actually was. Cat doors had been invented long before Newton. They weren't technically doors, though. They were called "cat holes," and were just that - small holes cut into doors that the cat came through. In an essay called "Philosophy and Common Sense," an unnamed country parson talks about visiting Newton's home. There he finds not one cat hole, but two. One is slightly smaller than the other. Newton's cat had just had kittens, and the joke was that this great genius had cut another hole in his door for the kittens, not realizing that they could simply follow their mother through the larger hole. What a dummy!

Most people say that the story was made up, although some people claim that a door at one of Newton's residences does show signs of having two cat holes in it. Whatever the actual origin, it's interesting to see how it changed with the times. When Newton was alive, or a recent memory, people laughed at how stupid a smart person could be. As time went by, and he was semi-deified, suddenly the legend shifted to make Newton seem even smarter than he was.